# Using the Many-Pole Self Energy (MPSE)

### From FEFF

To use the many-pole model self-enegy within FEFF, you must obtain an estimate of the loss function L(ω) = Im[ε−1(ω)]. The loss function can either be calculated, or found experimentally. A rough estimate can be calculated very quickly with the feff code using the OPCONS card, along with (optionally) the NUMDENS card. For more accurate calculations of the loss function, one can use codes based on the Beth-Salpeter equation or time-dependent density functional theory. Below is a list of codes which can perform these types of calculations:

• AI2NBSE

• YAMBO

• WIEN2K

• QUANTUM ESPRESSO

**Examples**

An input file that uses OPCONS to calculate the loss function and MPSE/SFCONV to calculate the resulting many-pole self-energy and many-body spectral function for a Cu K-edge XANES calculation follows:

TITLE Cu crystal * COREHOLE treatment RPA or FSR or NONE COREHOLE RPA * pot xsph fms path genfmt ff2x CONTROL 1 1 1 1 1 1 * Use the atomic database to form the loss function OPCONS * Use many-pole self-energy with density dependence MPSE 2 * Spectral function convolution SFCONV * This specifies HL exchange for both fine structure * and background, but in this case, the many-pole model * will be used, since the MPSE card is present. EXCHANGE 0 0 0 0 * Full multiple scattering with a cluster of 8.0 angstroms. FMS 9.5 * Shift the fermi-level back 1.5 eV and add 0.3 eV of experimental broadening. CORRECTIONS 1.5 0.3 * Self consistent potentials with cluster size of 5.0 angstroms. SCF 7.0 * Calculate XANES (changes energy grid used) XANES 5. * Calculate Debye-Waller factors for a temp of 10 K and Debye temp of * 315 K using correlated Debye model. DEBYE 10 315 0 ￼* Cu is fcc, lattice parameter a=3.61 (Kittel) POTENTIALS 0 29Cu 330.01 1 29Cu 331.0 ATOMS 0.0000 0.0000 0.0000 0 Cu_00 0.0000 -1.8050 1.8050 1 Cu_01 1.8050 0.0000 -1.8050 1 Cu_01 -1.8050 0.0000 -1.8050 1 Cu_01 1.8050 -1.8050 0.0000 1 Cu_01 -1.8050 1.8050 0.0000 1 Cu_01 . . .

This input file specifies that we want to calculate the Cu K-edge XANES of bulk Cu using the many-pole model dielectric function to calculate self-energy effects (MPSE) as well as multi- electron excitation effects (SFCONV). The file ‘loss.dat’ is required input for the many-pole dielectric function and in this case is obtained by specifying the OPCONS card. Note however, that the OPCONS card gives only a rough estimate of the loss function.

For a more accurate calculation, comment out the ”OPCONS” card and provide your own ‘loss.dat’ file, e.g. from an experimental measurement or an ab initio calculation. An example of a ‘loss.dat’ file can be found in ‘∼/jfeff-9.5.1/examples/MPSE/Cu/’. You can compare this to the ‘loss.dat’ file that will be generated from the OPCONS card in the example above or at ‘∼/jfeff-9.5.1/examples/MPSE/Cu OPCONS/’. You can also compare the resulting spectra. (Note that the 2 examples provided have many other differences. You should compare 2 calculations differing only in the use of the OPCONS card to study its effects.)